Our long range goal is to understand the molecular and cellular basis of tolerance (resistance) to the potentially damaging effects of hyperoxia on the lung. We will examine the mechanism responsible for the increase in catalase gene expression in neonatal rats, which exhibit endogenous tolerance to hyperoxia, and in adult rats in which tolerance has been induced by endotoxin treatment. In neonates as well as endotoxin-treated adult rats, the hyperoxia-induced increase in catalase activity is regulated, at least in part, at the post-transcriptional level by increased mRNA stability. Exciting new results indicate the presence in rat lung of a protein(s) that interacts with catalase mRNA in a redox-sensitive manner to form specific RNA-protein complexes. To understand the molecular mechanism responsible for regulation of catalase mRNA stability and to determine the cellular basis of the catalase gene response our Specific Aims are: 1. To test the hypothesis that in neonatal rat lung a redox-sensitive catalase mRNA-binding protein has a role in the regulation of catalase mRNA stability. In experiments designed to achieve aim 1 we will A) Delimit the region(s) of catalase mRNA required for protein binding, B) Identify and isolate specific protein(s) that bind to catalase mRNA, and C) Ascertain the role of these proteins in regulation of catalase mRNA stability. 2. To test the hypothesis that a catalase mRNA-binding protein(s) has a role in the regulation of catalase mRNA concentration in lungs from endotoxin-treated hyperoxia-exposed adult rats. 3. To use in situ hybridization of catalase mRNA in a quantitative ultrastructural analysis of the intact lung of neonatal and adult endotoxin-treated rats in order to identify cells whose catalase gene expression is responsive to hyperoxia. We believe our preliminary data and experience indicate the work is feasible and will provide fundamental new information about cellular and molecular mechanisms by which higher organisms adapt to oxidant stress.